The present invention relates to a color television camera (to be referred to as a color TV camera hereinafter) with a plurality of image pickup tubes and, more particularly, to a coil device arranged around the plurality of image pickup tubes to deflect electron beams generated inside the image pickup tubes.
Color TV cameras of the three-tubes type have been widely used for color television broadcast. In a color TV camera of this type, the optical image of a subject is separated into a red (R) image, a green (G) image and a blue (B) image. The separated images are respectively focused on targets of image pickup tubes for R, G and B primaries. The targets of these image pickup tubes are scanned with electron beams emitted by electron guns to obtain electrical signals corresponding to the images focused on the targets. These signals are synthesized to form a single color image.
In the three-tubes color TV camera, for synthesizing the output signals from the image pickup tubes which correspond to the primaries, registration adjustment is performed to avoid misregistration.
The registration adjustment is conventionally performed in the following manner. A registration chart is displayed on a monitor TV screen by the color TV camera and is superposed with images obtained by the R and G image pickup tubes. Horizontal and vertical deflection amplitudes, size alignments, and linearities of the R image pickup tube are adjusted to cause the R image to completely overlap the G image. Subsequently, the B and G images, obtained by the image pickup tubes, are superposed on the registration chart on the monitor TV screen. The B image is adjusted to overlap the G image in the same manner as described above. Thus, the registration adjustment, consisting of the 6 items above, is completed. In addition to the above items, two additional adjustment items such as skew adjustment and rotation adjustment may be added.
If all the items described above are checked in the registration adjustment, the adjustment cannot be completely performed. Assuming that the vertical length of the screen for raster scan is defined as 100%, even after registration adjustment, misregistration of about 0.05% occurs at the center of the screen and that of about 0.3 to 0.4% occurs in the peripheral portion. In this manner, misregistration is hard to eliminate.
In order to reduce misregistration, the following adjustment method is proposed in addition to the conventional adjustment items. Depending on the types of products, characteristics inherent to the image pickup tube do not necessarily correspond to deflection characteristics inherent to the coil assembly. Therefore, deflection distortions such as pincushion distortion and trapezoidal distortion occur due to the characteristics of the image pickup tube and the coil assembly, and a combination thereof. These distortions are considered to be a cause for misregistration. In order to correct these distortions, a method is proposed to superpose, on a vertical deflection signal, a correction signal which has a frequency component of a horizontal deflection signal as the fundamental wave. However, according to this method for correcting a distortion of an arbitrary horizontal line on the screen, the frequency component of the horizontal deflection signal is attenuated by an inductance of the vertical deflection coil. This is because the total number of turns of the vertical deflection coil is maximized to decrease power consumption by vertical deflection. In other words, the frequency of the vertical deflection signal in the NTSC system is 60 Hz. However, the number of turns of the vertical deflection coil is increased to an extent that its inductance is not important. Since the frequency of the horizontal deflection signal is higher than that of the vertical deflection signal, the current of the horizontal deflection signal can hardly flow through the vertical deflection coil. Thus, misregistration correction cannot easily be performed. The above problem will be described in detail with the accompanying drawings.
Referring to FIG. 1, assume that an image formed by an output signal from one of the image pickup tubes for primaries of the color TV camera is distorted like scanning line 10, while an image formed by an output signal of another one of the image pickup tubes for primaries is not distorted like scanning line 11. This results in misregistration. In order to correct the distortion to obtain a nondistorted image like the scanning line 11, the vertical deflection signal waveform shown in FIG. 2 must be corrected. FIG. 3 shows an enlarged part 20 of the vertical deflection signal waveform shown in FIG. 2. A broken line 30 indicates the signal waveform without correction, while a solid line 31 indicates the signal waveform on which a correction signal, including the horizontal deflection signal as the fundamental wave, is added. With the corrected signal, the distortion like the scanning line 10 shown in FIG. 1 is relatively easily eliminated.
However, a distortion of a scanning line 40 shown in FIG. 4 cannot be easily corrected. In order to correct the scanning line 40, a corrected vertical deflection signal waveform 51 is required as opposed to a nondistorted vertical deflection signal waveform 50. Further, since the vertical deflection coil has a large inductance, it functions as an integrating circuit for integrating the correction signal which has the vertical deflection signal as the fundamental wave. The waveform of a current flowing through the vertical deflection coil is an integrated waveform of the waveform of a voltage applied thereto. Referring to FIG. 5, the waveform of the correction signal alone is the current waveform 60 shown in FIG. 6A. However, in order to obtain such a current waveform in the vertical deflection coil, a voltage waveform 61, shown in FIG. 6B, must be applied to the vertical deflection coil. The waveform 61 is a differential waveform of the waveform 60. Therefore, the waveform 61 has a pulse of high level and narrow width in horizontal flyback period 62, and the pulsed portion is outside of the linear region of the vertical deflection circuit. Even if the above problem is eliminated, the vertical deflection coil has a large inductance and has narrow-band characteristics with respect to the horizontal deflection signal and its harmonic. For this reason, the frequency characteristic for the harmonic component is more damped than that for the horizontal deflection signal. Even if the ideal differential voltage waveform 61 of the current waveform 60 is applied across the vertical deflection coil, an integrated current waveform with attenuated harmonic component is obtained in practice. The current with the waveform indicated by the broken line 63 flows through the vertical deflection coil. As indicated by the broken line in FIG. 4, a terminal end 42 of the scanning line is properly corrected. However, an initial end 41 of the scanning line is distorted.
It is thus understood that the general deflection distortion shown in FIG. 4 cannot be easily corrected. If more complicated distortions are involved, a voltage of a more complex waveform must be applied across the vertical deflection coil. As a result, correction cannot be performed in practice, due to the frequency characteristic of the vertical deflection coil.
The above method has a further drawback; the current waveform is the integrated waveform of the voltage waveform. An integrated value between the voltage waveforms 64 and 65 shown in FIG. 6B must be zero. However, the pulsed portion cannot be properly integrated due to the linear region of the vertical deflection circuit and the frequency characteristics of the vertical deflection coil. In practice, the current waveform corresponds to the integrated waveform in which the integration value between the voltage waveforms 64 and 65 is not 0. In other words, a DC magnetic field is applied to the vertical deflection magnetic field, so that the image is vertically deflected. If misregistration occurs in this case, the adjustment of deflection distortions is complicated. Thus, misregistration is not desirable in this case.